Power line carrier frequency telephone system



June 6, 1950 Filed Oct. 11, 1946 FIG.

J. M; eARsTow ETAL 2,510,273

POWER LINE CARRIER FREQUENCY TELEPHONE SYSTEM 2 Sheets-Sheet 1 COUPLE/P REC.

FILTER cmp/Ef? TERM/NAL LINE CIRCUIT LINE CIRCUIT .JMBARSTOW /NVE/vO/s. A G LANG @@Cna ATTORNEY June 6, 1950 J. M. BARsTow ETAL RowER LINE CARRIER FREQUENCY TELEPHONE SYSTEM 2 Sheets-Sheet 2 Filed oct. 11. 194e S @www /NVENTORSJ M BA R5 7' 0W AGLANG AHORA/Ev Patented June 6, 1950 2,510,273 :POWER LINE CARRIER 'IRQUENCY TELEPHONE SYSTEM John M. Barstow, Mountain Lakes, N. J., and Albert G. Lang, Whitestone, Y., assign'ors `to .Bell 'Telephone Laboratories, Incorporated, New York, N. Y., a corporation .of New York Application October 11, 1946, -Serial No. 702,732

(Cl. 17H-2:5)

4 Clail'ns.

This invention relates to communication systems and particularly to telephone systems including subscriber stations arranged 4for carrier current operation over apower transmission line.

Objects of the invention are the provision of carrier telephone service over power transmission lines, the provision of means for recognizing reverting calls 'without delay and at the 'same time guarding against seizure of the carrier telephone line at vthe telephone control oiii'ce, and to insure the setting of the lcarrier line equipment `for a reverting call only vwhen a reverting call is being made.

This invention is a telephone system comprising' subscriber `stations arranged for carrier 'current operation over .a power transmission line and including means for setting the carrier terminal equipment at lthe telephone Ieiichangle for a reverting call without introducing a .delay .after each call during which no new call can be vinitiated over the power line. e

A feature of the Yinvention `is the ,provision 'in a telephone .exchange of carrier line terminal equipment connected "to a .power transmission line, the carrier terminal equipment vincludiiiga first carrier current receiverresponsive to signal and speech modulated carrier currents of afirst .frequency from a calling carrier station and including a second .carrier currentreceiver responsive to carrier .current of .a second frequency for setting the terminal equipment .for va .reverting call whereby .speech is 'transmitted directly Vbetween the calling and called stations on such .a call.

Another "feature of the invention is a telephone system comprising subscriber .stations .arranged for carrier current operation over a power 'transmission line and including an automatic 'telephone exchange provided with carrier terminal vequipment arranged to permit anew call Itobe initiated over the power line immediately 4following the termination of a call to or from any o'f the carrier telephone stations andarranjged so 'that release of the connection on a call "to or 'from a carrier ystation is under the joint control oi both the calling and called stations.

A Yclearand complete description of the v'inventionwill be facilitated by considering .a system in which the invention 'and its v,features are embodied, such la system being represented schematically in the drawings. The invention is, however, notY limited in its application 'to the systemv shown vbut is Vgenerally applicable to telephone systems.

v'Ihe dra'u'ying which consists oftwol gures, rep- .2 resents an .automatic telephone .exchange :serv-'- ing subscriber stations some of which are Yarranged :for carrier .operation .over a power transmission line.

Fig. 1 shows an 4a'utcmia'tic telephone exchange comprising a subscriber station l0, subscriber lines il I 'and .21., :and `.line .circuits l2 and .22, 'line -nder .switches LFI and LFZ, selector .switches Sl and S2, and connector switches lCI and :C2-'2. llig. s1 .further shows .a carrier Vterminal equipment CT individual to the .line 2.1 fand a :power transmission line PL to which the carrier terminal equipment CT is lcoupled by coupling means All.

Fig. i2 shows one of a plurality of subscriber stations connected to, and arranged for Vcarrier operation over, `the power line PL.

-The subscriber station `I is of the usual type provided for use. on lines terminating Vin an auto- -matic central cnice and includes a dial -or other impulse sender (not shown) for use in selective'- `T51-controlling the switches through which desired connections larie established. Each of the 'line circuits l`2 and 22 consists of two relays (not shown), 'a line relay and a cut-ofi relay. 'Ihe V line vinder, selector and connector Switchesmay be oi the two-motion step-by-step type disclosed in Automatic Telephony by Smith and Campbell, pages 53 to 65, inclusive, of the second 'edition. Each lof the line finder and selector switches is represented in the ldrawing by a set of brushes and a single set -of terminals,A and each-of the connector lswitches is represented by aset of'brushes and two sets of terminals. The relays and other apparatus associated withthe line finder, selector and `connector `switches are .represented by broken lines. Reference maybe `had to "the -patent v.to R.. L. Stokely No 1,799,654, granted April 7, 1'931, for a disclosure 'of a line circuit ,and a line nder of'fthe 'step-by-step type. The ringing code .generator 2? comprisesa source of ringing current and means for .interrupting the yringing; ,supply yleads as required to produce a plurality of different ringingcodes for signaling subscriber stations. q Reference may be had 'to the patent to DuguidNo. 2,225,907,granted December 24, 1940, for disclosure cfa ringing codegenerator and to the Vpatent to iKoechling 'No'. 2,289,503, granted July 14, '1942, for a disclosure of a connector switch arranged for ten-,party coded ringing.

The carrier 'terminal equipment CT comprises a hybrid coil or 'group of vtransformer windings starranged totra'nsmitsignal and voice currents to and vfrom line 2|. Theresis'to'r 3| and condenseriBZ constitute a network connected to the right middle pair of windings to balance the line 2|. The upper pair of windings receive voice frequency currents from the amplier-demodulator 60 and the lower pair of windings transmits voice frequency currents to the input of amplifier-modulator 49. The power line PL is one phase of a power transmission line which serves a :plurality of power and light customers some of which are equipped with carrier telephone equipment as shown in Fig. 2. The power line is connected through coupling means 44, and conductors 43 to the receiving band lter 59 associated with amplier-demodulator 99, to the receiving band filter 49 associated with amplifierdemodulator 56 and to the transmitting band filter 4I associated with amplifier-modulator 4I). The amplifier-modulator 49 is arranged to transmit through filter 4 I, carrier current of frequency FI, modulated at a signaling frequency or modu lated by voice currents incoming to hybrid coil 43|l from line 2 I. The amplifier and demodulator 6U receives, through lter 59 from the power line PL, carrier current of frequency F2 modulated by voice and signal currents from any one of the carrier telephone stations served by the power line PL. The amplifier-demodulator 59 receives through filter 49 from power line PL, carrier current of frequency FI from any one of the carrier telephone stations served by the power line PL on a reverting call, that is, a call :between two of the carrier telephone stations connected to power line PL.

The carrier terminal equipment CT further comprises relays 34 and 35, controlled by ringing current incoming over line ZI to energize the amplifier-modulator 49 and thereby eiect the transmission of carrier current impulses corresponding to the ringing code received by relay 34 from line 2|. The carrier terminal equipment further comprises relays 6E, 62 and 63 controlled by amplifier-demodulator 66 responsive to signals transmitted over line PL by carrier current of frequency F2; and comprises relays 5l, 52, 53 and 54 controlled by amplifier-demodulator 59 responsive to signals transmitted over line PL by carrier current of frequency FI. The function of each of these relays is hereinafter described in detail.

Each of the plurality of telephone subscriber stations is similar to the station shown in detail in Fig. 2 and is connected through individualcoupling means 69 to the power line PL. rThe station includes a receiver 84 and transmitter 85 combined in a handset which normally rests on the hook 83. A ringer I9 responds to incoming ringing signals and the dial 86 is used in controlling the automatic switches in the central oce to complete desired connections. The carrier equipment at each station comprises a receiving band filter II tuned to the carrier frequency FI, a receiving band filter l2 tuned to the carrier frequency F2, amplier and demodulator apparatus 13, a ringing relay 'i6 controlling the ringer 19, amplifier and modulator apparatus B8, a transmitting band filter 89, a frequency control relay 99, oscillator crystals 9i and 92, a transmitter cut-ofi' relay 93 and a three-element gas-filled discharge tube 95. The power supply for the amplifier, modulator and demcdulator apparatus includes the transformer 65, connected to the power line directly or through an intermediate transformer, and a rectifier 6l. Other transformer windings (not shown) are provided for supplying the laments of vacuum tubes and whatever operating potentials are required.

A key RV is provided for use in making reverting calls. Each of the other carrier telephone stations connected to power line PL is similar to the station which is shown in detail.

The carrier terminal equipment CT, power line PL, and the carrier telephone stations of Fig. 2 constitute a power line carrier system similar generally to the system disclosed in the copending application of R. C. Edson and J. W. Emling, Serial No. 653,254, filed March 9, 1946, Patent No. 2,481,915, granted September 13, 1949, to which application reference may be had for discusion of the carrier frequencies suitable for use in such systems and for the disclosure of details of the coupling means between the power line and the carrier equipment CT and carrier telephone stations.

Call from .station 10 to carrier telephone station Assume that a call is initiated at station I0, that the line I I is connected by a line nder LFI to a selector SI, that the selector SI is selectively controlled by dial impulses from station I6 to select a group of trunks and an idle trunk therein associated with the connector CI, and that the connector CI is selectively controlled by dial impulses from station I0 to select the line 2 I. Assume further that the line 2| is idle when tested by the connector CI and thata ringing code is selected responsive to dial impulses from station A and that this ringing code is transmitted through the brushes of connector CI over the conductors of line ZI, through condenser 33, the Winding of ringing relay 34 and the left back contact of relay 62. Relay 34 is operatively energized during each ringing interval of the ringing code, and the operation of relay 34 closes a circuit for operating relay 35. The operation of relay 35 opens the normally closed connection between the lower windings of hybrid coil 39 and amplifier-modulator `40 and connects the positive pole of battery B2 through resistor 36 to energize the amplier-modulatorw and thereby transmit carrier current of frequency FI through filter 4i, conductors 43, coupler 44, over power line PL, to each of the carrier telephone stations served by the power line PL. The carrier current thus transmitted is modulated at a 16-cycle rate by breakdown of discharge tube 3l every time condenser 38 is charged through resistor 36 to the breakdown potential, the capacity of condenser 38 and the resistance of resistor V36 being such that the tube discharges 16 times per second.

At each of. the carrier telephone stations, the l-cycle modulated carrier current of frequency FI incoming over power line PL is transmitted through a coupler 69, conductors l0, and aback contact of relay and throughreceiving lter 'II to the ampliiier-demodulatorl The output conductor Yof demodulator I3 is connected through resistor 14 to the Winding of relay 16. A condenser 15 connected in parallel with the winding of relay 'I6 renders this relay somewhat slow in operating so as to tend to prevent its operation responsive to interference currents such as static. The output conductor of demodulator 'I3 is further connected through the condenser SI and the off-normal contact of dial 85 to 'telephone receiver 84 in parallel with resistor92. While the telephone handset is in normal position on hook -83, the receiver 84 and resistor 82 are short-circuited ata back contact of springs controlled Iby hook 83. The operation of relay 16, responsive Vto ringing code impulses of carasi-ogre the nlc'wer winding of transformer t5, "through rectifier `61, conductor '68, resisto-r BU, front con tact 'of relay 16, condenser 11, windings of ringer l19, to ground at a back 'contact yof springs conu trolled by hook 83. The ywindings o' ringer 19 are thereby energized while condenser 11 is charging. Since the Aincoriiiflg carrier 4current of frequency Fl is nflodulated at l cycles, Yrelay 16 is alternately operated and released at a 16n cycle rate during each ringing interval of the ringing code. Each time relay 16 releases, condenser 11 discharges through resistor 18'; and each time relay 16 reoperates, condenser 11 is charged through ringer 19. Thus the 'ringer 'at Veach of the carrier vtelephone lstations yis actu'- lated 'in accordance with 'the vringing code of the particular station which lis being called.

v When the handset is removed 'from Athe `hook 83 at the called station, the circuit through Yringer 19 is opened, the short-circuit across the receiver 84 is opened and a positive operating potential is supplied to the amplier-modulator 88, this supply path being traced from conductor 68, througha front contact of springs controlled by hook 83 and through the back Contact of relay 93. The amplifier-modulator -88 is thereby energized and, with the oscillator vcrystal '92 operatively connected through the inner left back coritact of relay 90 'in the oscillator circuit of the modulator, carrier current of'frequency F2 lis generated and transmitted through the transmitting `band filter 89, conductors 10, and coupler 69 to the power line PL, and vover the fpower line PL to each of the l'other carrier stations and also to the carrier terminalequipment CT. The carrier current of frequency F2 fdoes hot reach the demodulator 13 at each of the other carrier stations because the input circuit of lter 12 of each-station is open as long'as 'the handset is in normal position on hook 83. The carrier current of frequency F2, transmitted over Yline PL to the car- 'rier terminal equipment CT, passes through coupler 44, conductors 43 and receiving-band filter 59 to the .input circuit of -ampliiier-demodulator 60, and the rectified carrier current'outpu-t effects the energization of relay 6I. Theoperation of relay 6| closes a circuit for operating slow-torelease relay, and closes a'di'rect'current path through the left middle pair of vwinding'sfof hybrid `coil 30 across line `-ifl thereby tripping vthe ringing in the connector Cl. The opening of the ringing circuit and closingof thetalking vcon-'- nection through connector Cl is'effected in the usual and well-known manner. The operation of relay 62 opens the circuit jthroughthe winding of relay 34 so that relays 34 and 35 release, if operated. The 'operation Vof relay 62 connects the positive pole of battery B2 4through a front contact off relay 62 and a back contact 'of relay 63 to the amplifier-modulator 40; and, with relay 35 released, the lowerpairof'windings of 'hybrid coil 30 are connected to the input of ampliermodulator 40, whereby carrier rcurrent "or fre-'- quency FI is modulated by voice currents from line 2l and the modulated carriercurrent is transinitted through filter 4|, over power line PL, through lter 1I to the amplier-demodulator 13; operating relay -16,fand at the 'called station, at which the handset has been -removedto V'answer the call, the output of vdemodulator 13 is transmitted to the receiver '184. Voice current n from the transmitter 85 of the handset fof the called station -is transmitted through Lcoil -81 :to the `Vinput circuit of ambli'fiemdulitdi 88,

therein to modulate the carrier yourrient "of free due'ncy F2 which fis being 'transmitted foyer the Power line. 'The voice rin'odulatt'e'd carrier current of :frequency F2 transmitted Iover the .power line passes through --i'llter Y59 to demodulator 60 fand the voice current voutput is transmitted 'through hybrid coil 3Q ever line 23|-, through the 'cone nectens'elector Vand line finder switches over-line yIl to the Y calling station. 5

When the handset iis replaced on hook l83 at the called station, receiver v84 is vagain 5"shortcircuited'and the ampli'ieremodulat'or VB8 is deenergiaed, causing the successive -release of relays V6| 'and 62 ofthe carrier terminal equipment The release of relay 6I opens the vbridge facA vss line 2|, thereby releasing the answering super- Visory relay (not shown) Yin the connector Cl. The release of relay 4B2 disconnects battery B2 from ampliiier-modulator 40 so fas to terminate the vtransmission of carrier current of frequency FI over the power line, 'therebycausing the re'- lease of relay le lat each of the carrier stations. When the calling station releases the connection, the `connector Cl, selector Sl and line 'finder LFI are restored to normal in usual and well known manner.

Call from carrier telephone station 4to non-carrier station,

Assume next that one of the carrier telephone stations connected to power line PL removes the handset from the receiver hook to initiate a call. The amplier-modulator 88 is Vthereby energized to transmit carrier 'current of frequency F2 over power line PL in Athe manner above described, thereby causing the operationofwr'elays 6l and 62 of carrier terminal equipment CT. The operation of relay 'Eil closes the bridge across line 2l whereby the line relay (not shown) 4of'line circuit V.22 is operated to start an idle line finder such yas LF2 to connect with Jthe calling line, and the line v2l is'thenconnected through the'brus'h'es of line lnder LFZ to the vassociated selector. S2. The operation of relay 62 energizes 'theiampliner-modulator 4l] and dial tone is transmitted in usual manner from selector 'S2 over line 2|, through hybrid coil 3B to the input of amplifiermodulator Ml; and dia-l tone modulated carrier current of frequency Fl is transmitted through filter 4l over power line PL 'to the carrier telephone stations. The carrier-current of 'frequency FI effects the operation of relay -16 at each 'oi the carrier stations; and, at the calling vcarrier station, the dial tone is transmitted through'the receiver 8d to indicate that the dialing of the number of the telephone station with which cohnection is desired may be started. When the calling party dials the directory Vrnumber of the called station, the dial impulses modulate the carrier current of frequency F2 transmitted through lter 8S over the power line 'through lter 53 of carrier terminal equipment CT to the input of amplier-modulator vlill. The receiver 84 is disconnected 'from amplifier-demo'dulator 13 while the dial is off normal.

Each dial impulse transmitted from the calling station over the power line causes the release of relay ci. The release of relay lil, 'responsive to the rst impulse of each train corresponding to a digit of the called subscribersnumber, closes a circuit for operating relay 63. Vl'telay-63 is slowe to-release and remains operated during vthe re'- sponse of relay 5| to all of the impulsesof fa train. The operationjof relayf6'3, deenergizes the amplifer-modulator 40 and -'energizes-the ampli` iier-demodulator 50 to enable the operation of relay I in case the call being made is a reverting call. Each releaseof relay 6| responsive to dial impulses opens the bridge across line 2 I, thereby repeating the dial impulses to selectively control the operation of selector S2 and a connector C2 to extend the connection to the called line and to select the ringing code required for signaling the called station. When relay 63 releases at the end of each train of impulses received by relay 6 l, the amplifier-modulator 46 is reenergized and the amplier-demodulator 50 is deenergized.

Assume irst that the called station is not one of the carrier stations connected to the power line PL. When the call is answered, the connector C2 opens the ringing circuit and closes the talking ,circuit in usual and well-known manner. Talking currents are transmitted through hybrid coil 36 and over the power line in both directions. The connection is under the control of the calling carrier telephone station. When the handset at this station is restored to normal position on the hook 83, the amplifier-modulator 88 is deenergized causing the successive release of relays 6| and 62 of the carrier terminal equipment CT. The release of relay 6| opens the bridge across line 2| and the release of relay 62 deenergizes the amplifier-modulator 46. The opening of the bridge across line 2| causes the release of the line relay (not shown) of the connector C2 and return of the selector S2 and line finder LF2 to normal in usual manner. The connector C2 is also restored to normal in usual manner as soon as both the calling and called stations have released the connection.

Call between carrier stations Assume next that the call initiated at a carrier telephone station as above described is a call to another carrier station connected to power line PL and equipped to operate with the same frequencies as shown, such a call being known as a reverting call. After the connector C2 is selectively operated to connect with the terminals 25 of line 2|, it makes the usual busy test and nds the called line 2| busy; and busy tone is thereupon transmitted from the connector through the selector S2 and line finder LFZ, over line 2|, through hybrid coil 36, amplier-modulator 46, filter 4|, power line PL, filter 1|, amplifier-demodulator 13, and condenser 8| to the telephone receiver 84 at the calling carrier station.

Upon hearing the busy tone, the calling subscriber operates the non-locking key RV and holds the key in operated position while the called station is being signaled. Operation of the RV key closes a circuit through the right Winding of relay 96 and a circuit through the winding of relay 93 in series with the thermistor 94. lRelay 93 operates immediately but relay 93 is marginal and does not operate for a predetermined interval of time, for instance, about .l second. l/Vhen thermistor 94 is heated to the point at which the resistance is suiciently decreased, the current through the winding of relay 93 effects the operation of relay 93. The aforementioned operation of relay 96 opens the input to filter 1|, closes the input to filter l2, inserts crystal 9| in the oscillator circuit of modulator 88 in place of crystal 92 thereby to change the frequency of the carrier current transmitted through lter 89 from frequency F2 to frequency Fl, closes a locking circuit through the right winding of relay 96 independent-l of key RV, connects the positive potential supply conductor 68 to amplifier-modulator 88 independent of the switchhook 83, connects conductor 68 through resistors 91 and 96 to the control anode of gas-filled discharge tube 95, and connects ground to the main anode of tube 95. The ampliner-modulator 88 now transmits carrier current of frequency Fl for about .1 second until relay 93 operates. When relay 93 operates, it disconnects supply conductor 68 from amplifier-modulator 88, thereby terminating the transmission of carrier current of frequency FI therefrom.

When the frequency of the carrier current incoming over power line PL to the'carrier terminal equipment CT is changed from frequency F2 to frequency FI, relay 6| releases thereby opening the bridge across line 2l and causing the operation of relay 63. The opening of the loop across line 2|, tells the connector C2 to make a reverting call test to determine whether the call is a reverting call; and, if so, the line flinder LFZ and selector S?. are restored to normal but the connector C2 remains connected to the line 2|. Reference may be had to the patent to H. Hovland, No. 1,849,087, granted March 15, 1932, for disclosure of a connector arranged to make the aforementioned reverting call test. The operation of relay 63 deenergizes amplifier-modulator 40 and energizes amplifier-demodulator 5U so that the impulse of frequency FI transmitted from the calling station responsive to the operation of the reverting call key RV causes the operation of relay 5|. The operation of relay 5| causes the operation of relay 52. When relay 5| releases, at the end of the impulse of frequency Fl, relay 52 is held operated and relay 53 is operated by the current in a circuit traced from battery through the winding and inner left front contact of relay 52, winding of relay 53 and back contact of relay 54. The operation of relay 52 closes a circuit for -holding relay 63 operated and bridges the winding of ringing relay 34 in series with condenser 33 across line 2| so that relay 34 will respond to ringing current incoming over line 2| from connector C2. The operation of relay 53 disconnects battery B2 from amplier-demodulator 50, opens one path for energizing amplifiermodulator 46 so that energization of amplifiermodulator 40 is under the control of relay 35, and prepares a circuit path for operating relay 54 as soon as either of relays 5| or 6| is again operated. By the time relay 34 is rst operated by ringing current from connector C2, relay 62 will have released so that when relay 35 operates, the positive pole of battery B2 is connected through the outer lower front contact of relay 35 and right back contact of relay 62 to reenergize amplier-modulator MJ and thus effect the transmission of 16-cycle modulated current of frequency F! over power line PL to each of the carrier telephone stations connected to line PL. During the silent intervals of each ringing code cycle, during the silent intervals between cycles and after ringing is tripped, relays 34 and 35 will be deenergized, the release of relay 35 being effective to deenergize amplifier-modulator 40 and to energize amplier-demodulator 50. When the call is answered at the called carrier telephone station, carrier current of frequency F2 will be transmitted therefrom over line PL, causing the operation of relay 6| of the carrier terminal equipment CT. The operation of relay 6| effects the reoperation of relay 62, the operation of relay 54 and the closing of the bridge across line 2| to trip the ringing in the connector C2. Relay 54 locks independent of relay 53 and the. operation of relay 5 4 eglises the release of relays 5 2; and 5 3 lt isti-1e function. ofrelay 5 4 tolinsure the leerer iz or amplifier-meduetorl while atoplriier modulator 50: is ener,- szed. by hold-ine relay t3.. operated.. When the rinsing stops'the o iris subscriber releases the Rv key whereupon lay 9.3 releases. and amplieremodulotor |311v is. reerrereited to transmit earrier current of frequency Fl. The carrier curreit of' frequency thus transmitted from the telling station over line EL. eir'eots the operation of relay 5 I of the carriertermlnal equipment CT: AS lons; es. either off relays. it and 6| remains olocrateri;v relay .5,4 is hel serrated and the bridge line. 2! iS maintained So that release of the oourieetor- G2.- aweits the replacing of the reoeiver on the heels et both of the. tolling ond eolie@ stations- Voice, modulated carrier current of frequency FZ: is transmitted from the called station overV line PL to the, calling station; and voice modulated. Gefrier current of frequency Fl is transmitted from the calling station over line PL to the called station. When the receiver is placed n the hook at the called station., the ampliermodulator thereat is deenergized causing the release 0f relay 6| of, tit-1e.y carrier terminal equipment CT. When ther handset is placed on the hook at the calling station, the crystal 92 is included in the oscillatory circuitJY ofv the trailsmitting means in place of crystal Sol to cause the transmission of carrier current.. of frequency F2 in place of carrier current of frequency Fl Relay 5| or carrier `terrrlilnal eeupment CT is released and relay E! recher-ated when the frequenoy ofv the oor-tierA eur-rent. transmitted over Powerline Pt from theI calling station is thus changed from `frequenti' El to ,frequency F2. The replacing of the handset opens a short-,ciroirouit votioss. resistor 9 8. and condenser 99; and Condenser 981s Charged.. iii a circuit .from eositive potential supplyconductor 6d, through the next to the outer right frcnt contact of relay 90., resistor 9 1, resistor S8. end condenser 99 to ground. The potential across the control electrodes of gas-lled tube 9 5 is thus increased as condenser` 99 charges for about .5 second; whereupon the tube bre-ales down and a circuit is closed through the left winding of relay 9|! and the main discharge gap oi tube 95, The left winding of relay 90 is thus energized in opposition to the right winding to cause the release of relay 90. When relay 9 0 releases at the calling station, the amplifier-modulator 88 thereat is de: energized. causing the release of relay 6| of carrier terminal equipment Cr. with both of relays 5I and 6| released, the fbridge across line 2| is Opened causing the connector G2 to be restored to normal; and the holding circuit vfor relay 54 is opened causing the release of relays 54 and 63 in succession,

The calling subscriber may, on a, reverting call, release ley RV and hang up the receiver before the oel! has been answered at the calice station- .Iii Such a oase, relay S3 releases when the Ry key is. released and carrier current of rrequency Fl is then transmitted from the calling station until the handset is restored to normal. When the handset is replaced, carrier current of frequency F2 is transmitted from the calling station for .5 second as described in the preceding paragraph. When carrier current of frequency FI is transmitted over the power line responsive to the release of the RV key, relay 5l is not operated because at this time the amplier-demodulator 50 is not energized. When the impulse ot carrier circuit offreeuenoy F2 is transmitted over the power line responsive tothe harig-un of the receiver, relay 6,1 operatesV and, then releases. The operation of relay 6l closes the bridge across line 2|, causes the operation of relays 6 5! and 5 4 and the release of relaysl 52,- and 5 3, The closure across line 2| trips. the ringing in the connector C2, and when relay 6| releases, relays 5.4 62 and 63 release. If the impulse of carrier current of frequency F2- is inoperative to trip ringing, ringing continues until the receiver is removed at one of the carrier telephone stations.

What is claimed is:

1f. In a telephone system comprising subscriber lines, switching means for interconnecting calling and called subscriber lines, a power trans.- mission line and carrier terminal equipment. interconnecting one of said subscriber lines and said power line, a, plurality of telephone stations connected to said power line, each of said stations comprising means for transmitting over said power line voice and signal modulated carrier current of one of two Icarrier frequencies, means for receiving over said power line voice and signal modulated carrier current of the other of said two carrier frequencies, and means come prising a key for changing said transmitting and receiving means so as to transmit over said power line carrier current of said other frequency and receive over said power line carrier current of said one frequency, said carrier terminal equipment comprising means for transmitting over said power line to said stations carrier current of said other frequency modulated by voice and signal currents received from said one subscriber line, normally operative means for receiving voice and signal modulated carrier current of said one frequency from said power line and'fo'r transmitting the demodulated voice currents and signals over said subscriber line, normally inoperative means for receiving over said power line carrier current signals of said other frequency, and relay means controlled by said normally operative receiving means for rendering said normally inoperative receiving means opera# tive responsive to the termination of carrier current of said one frequency over said power line.

2. In a telephone system comprising subscriber lines, switching means for interconnecting calling and called subscriber lines, a, power transmission line and carrier terminal equipment interconnecting one of said subscriber lines and said power line, a plurality of telephone stations connected to said power line, each of said stations comprising means for transmitting over said 4power line voice and signal modulated carrier current of a crst one of two carrier frequencies, means for receiving over said power line voice and signal modulated carrier current of the other of said two carrier frequencies, and means com prising a key for changing said transmitting and receiving means so as to transmit over said power line carrier current of said other frequency and receive oyer said power line .carrier current of said 1jr-st frequency, Sad carrier terminal equipment comprising means for transmitting over said power line to said stations carrier current of said other frequency modulated by voice and signal currents received from said one subscriber line, normally operative means for receiving voice and signal modulated carrier current of said first frequency from said power line and for transmitting the demodulated voice currents and signals over said subscriber line, normally inoperative means for receiving over said power line carrier current signals of said other frequency, relay means controlled by said normally operative receiving means for rendering said normally inoperative receiving means operative responsive to the termination of carrier circuit of said first frequency over said power line, and relay means controlled by said normally inoperative receiving means for disabling said transmitting means-for disabling said normally inoperative receiving means and for enabling the further energization of said normally inoperative receiving means.

3. In a telephone system including subscriber lines, a power line and a plurality of subscriber stations arranged for carrier current operation over said power line, carrier terminal equipment connecting one of said subscriber lines to said power line, said equipment comprising transformer means connected to said one line for transmitting voice frequency currents to and from said one line, normally inoperative carrier current transmitting means for transmitting over said power line voice and signal modulated carrier current of one of two carrier frequencies, relay means responsive to ringing current received over said one line for energizing said transmitting means and for modulating at a desired rate the carrier current of said one carrier frequency transmitted over said power line, normally operative receiving means tuned to receive carrier current of the other of said two frequencies, means including said transformer means and said normally operative receiving means for transmitting voice frequency currents over said one line, relay means controlled by said normally operative receiving means to close a bridge across said one line, to energize said transmitting means and to disable said ringing relay means, normally inoperative receiving means tuned to receive carrier current of said one carrier frequency, relay means controlled by said normally operative receiving means for enabling said transmitting means responsive to carrier current of said other frequency incoming over said power line, for disabling said transmitting means responsive to the termination of carrier current of said other frequency over said power line, and for ener gizing said normally inoperative receiving means responsive to the termination of carrier current of said other frequency over said power line, relay means controlled by said normally inoperative receiving means responsive Vto an impulse of carrier current of said one frequency for enabling the response of said ringing relay means to ringing current thereafter incoming over said one line, thereby to reenergize said transmitting means to transmit carrier current of said first frequency modulated at said desired rate responsive to ringing current and for again deenergizing said normally inoperative receiving means at the end of said impulse, relay means controlled by said normally operative receiving means responsive to carrier current of said other frequency incoming over said power line following receipt of said impulse of carrier current of said one frequency by said normally inoperative receiving means for reclosing said bridge across said one line, for disabling said transmitting means, for maintaining the bridge across said one line and for energizing said normally inoperative receiving means and maintaining the deenergization of said transmitting means as long as carrier current is being received over said power line by either one of said receiving means.

4. In a telephone system, subscriber lines, a power line, a plurality of subscriber stations arranged for carrier current operation over said power line and carrier terminal equipment connecting one of said subscriber lines to said power line, each of said subscriber stations comprising receiving means normally tuned to receive carrier current of one of two carrier frequencies, relay and ringer means normally responsive to ringing signal modulated carrier current of vsaid one frequency incoming over said line, a telephone receiver, a receiver hook, a set ofA contact springs controlled by said receiver hook including a contact normally disabling said receiver and a contact for disabling said ringer when the receiver is removed from said hook, transmitting means including oscillator means for transmitting carrier current of either one of said two frequencies, a telephone transmitter, means connecting said telephone transmitter to modulate the carrier current transmitted by said transmitting means over said power line, a dial for modulating the carrier current transmitted by said transmitting means over said power line, means for energizing said transmitting means when said receiver is removed from the receiver hook to initiate a call to transmit carrier current of one of said frequencies, a non-locking key for use on an outgoing call to another of the stations connected to said power line, a frequency changing relay controlling the frequency of said oscillator means and controlling the frequency to which said receiving means is tuned, means comprising said key for operating said relay while said receiver is removed from the receiver hook to terminate the transmission of carrier current of said one frequency and transmit carrier current of the other of said frequencies, means for deenergizing said transmitting means a predetermined interval of time after said relay operates and for maintaining the deenergization of said transmitting means until said key is released, and means for holding said relay operated after said key is released until said receiver hook is restored to normal.

`JOI-IN M. BARSTOW. ALBERT G. LANG.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,289,048 Sandalls, Jr. July 7, 1942 2,294,905 Honaman Sept. 8, 1942 2,408,085 Meacham Sept. 24, 1946 

